The invention relates to a ground fault protection circuit for enhancing the safety in a neon transformer which is used for lighting a neon tube or an argon tube.
FIG. 1 shows a ground fault detection circuit for a conventional neon transformer of the kind described. A leakage transformer (neon transformer) 11 includes a primary winding 12, one end of which is connected through a switch 13 to an input terminal 14 and the other end of which is connected to an input terminal 15. A pair of secondary windings 16, 17 have starting ends which are connected together at a junction 20, which is connected to a ground terminal 18 of a transformer casing 36 or thus to the casing 36. The ground terminal 18 is connected to the ground. Terminal ends of the both secondary windings 16, 17 are connected to output terminals 21, 22, across which sign lamps 23 such as neon tubes or argon tubes are connected. An a.c. power, for example, a commercially available power is applied across the input terminals 14, 15 and is stepped up by the transformer 11 before it is applied to the sign lamps 23 to light them.
A detection circuit 10 is provided which detects a ground fault which would occur if the sign lamp 23 or its wiring should come into contact with the casing 36 or a tower on which the sign lamps 23 are mounted and disconnects the input a.c. power. Specifically, the detection circuit 10 includes, as part thereof, tertiary windings 25, 26 disposed close to and magnetically coupled to the secondary windings 16, 17. Normally, the tertiary windings 25, 26 are disposed on a magnetic core on which the secondary windings 16, 17 are disposed in a manner such that a layer of insulating material having a high withstand voltage on the order of 6000-7000 V is located interposed between the tertiary windings 25, 26 and the lowermost layers of the secondary windings 16, 17 to provide a high electrical insulation therebetween while allowing a satisfactory magnetic coupling between the secondary windings 16, 17 and the tertiary windings 25, 26.
At their one end, the tertiary windings 25, 26 are connected together in an inverse phase relationship so that their induced voltages cancel each other, while at their other end, the tertiary windings 25, 26 are connected to an input of a rectifying and smoothing circuit 27, the output of which is connected through a Zener diode 28 across a parallel circuit comprising a resistor 31 and a capacitor 32 and which is in turn connected across the gate and the cathode of a triac 33. The triac 33 is connected across the input terminals 14, 15 through a relay drive coil 34, which when energized, controls relay contacts that define the switch 13.
Under a normal condition, voltages induced across the tertiary windings 25, 26 are substantially equal in magnitude but are opposite in phase, whereby an input voltage to the rectifying and smoothing circuit 27 is nearly zero. However, upon a ground fault of the signal lamps 23 or the wiring thereof, one of the secondary windings which is associated with the ground fault will be short-circuited, causing a substantially decrease in the induced voltage in the tertiary winding which is coupled with this secondary winding to allow the full induced voltage in the other tertiary winding to be applied to the rectifying and smoothing circuit 27. This voltage is rectified and smoothed and an increase in the rectified and smoothed output voltage turns Zener diode 28 on, with consequence that the triac 33 is turned on to energize the relay drive coil 34 to open the switch 13, thus interrupting the supply of the input a.c. power to the transformer 11. The switch 13 comprising the relay contacts is thrown to the normally open position NO, whereby the holding current to the relay drive coil 34 flows through the relay drive coil 34.
It will be noted that in the described conventional circuit, the pair of tertiary windings are used and disposed below (or inside) the lowermost layer of the pair of secondary windings with a high withstand voltage insulation. The provision of the tertiary windings requires time and labor, reducing the production efficiency of the neon transformer, in particular, for a step-up transformer in which the discharge tubes are lit by a high frequency power generated by an inverter. A reduced physical size of the transformer presents difficulties in winding the tertiary windings and the insulating film and treating lead wires.
Protection against a secondary ground fault of such a neon transformer is disclosed in FIG. 3 of the U.S. Pat. No. 5,847,909 issued Dec. 8, 1998, where the protection circuit does not employ tertiary windings, but uses an increased number of parts and results in a complicated arrangement, which renders it difficult to utilize a conventional box for containing a neon transformer. A ground fault protection circuit for a power supply transformer is also disclosed in U.S. Pat. No. 6,040,778 issued Mar. 21, 2000. However, this circuit also has a drawback of using a complex arrangement.
For a neon transformer, it is mandated by legal regulation that the ground terminal be always connected to the ground in view of the safety consideration. However, there is a likelihood that a dealer who undertakes constructing a neon sign or a neon tower which uses neon lamps may forget the work of connecting the ground terminal 18 to the ground. A no-ground connection protection circuit which detects such condition during use to interrupt the supply of the a.c. power is proposed and shown in FIG. 2 of above cited U.S. Patent. However, this no-ground connection protection circuit again requires an increased number of parts and results in a complicated arrangement, rendering it difficult to utilize a conventional box for containing a neon transformer.
It is an object of the invention to provide a ground fault protection circuit for a discharge tube lighting circuit having a reduced number of parts and a simple arrangement and which can be readily manufactured.
The present invention relates to a ground fault protection circuit for a circuit arrangement in which an input a.c. power is converted into a d.c. power in a rectifying and smoothing circuit, the d.c. power is then converted into a high frequency power by an inverter including the primary winding of a step-up transformer, and the high frequency power is stepped up by the step-up transformer to light a cold cathode discharge tube connected across the opposite ends of a secondary winding thereof. According to a first aspect of the invention, a first and a second coil are disposed so as to be threaded by lead wires extending from the opposite ends of the secondary winding of the step-up transformer and are connected to a detection circuit which detects a ground fault occurring on the output side of the secondary winding. An interruption circuit is controlled in response to a ground fault detection output from the detection circuit, thereby interrupting the supply of the high frequency power to the secondary winding.
According to a second aspect of the invention, a resistive element is connected between the magnetic core of the step-up transformer and a point of common potential, and a detection circuit is connected across the resistive element to deliver a ground fault detection output whenever a high frequency voltage across the resistive element becomes equal to or greater than a given value, thus allowing an interrupter circuit to be controlled by the ground fault detection output to interrupt the supply of the high frequency power to the secondary winding.